The present invention relates to a signal processing circuit for use in a color video signal recording and/or reproducing apparatus. The phase angle of a carrier chrominance signal is sequentially rotated or shifted by 90.degree. for every horizontal scanning period. More particularly, the invention relates to a signal processing circuit for producing a signal for shifting the phase of the carrier chrominance signal, of the color video signal, by 90.degree. in the recording and/or reproducing apparatus.
In a conventional video signal recording and reproducing system, the reproducing head simultaneously reproduces video signals recorded on mutually adjacent video tracks. As a consequence of a tracking error which occurs at the time of reproducing, beat disturbances arise in the reproduced picture. Accordingly, in order to prevent a generation of such beat disturbances, video tracks are recorded on a medium with unrecorded areas, called .-+.guard bands" between them.
The beat disturbances decrease as the width of the guard bands increase. There is a poor efficiency of the recording medium. Therefore, from the standpoint of improving the utilization efficiency of a recording medium, it is desirable to either reduce or eliminate the guard bands.
Accordingly, a known system uses recordings and reproducing heads having an obliquely directed gap. Thus, it is possible to reproduce signals in a manner which prevents beat disturbances, even when the recording is made without guard bands. In this system, two heads have gaps with azimuths which are oppositely inclined, with respect to the perpendicular of the scanning direction followed by the heads, for example. These heads are used alternately to record video signals in contiguous tracks, without guard bands, on a recording medium. In the reproducing system, a track is reproduced without loss if the recording head has the same azimuth as the recording head had. However, the contiguous track is recorded by a head having an azimuth of opposite direction, and the reproducing loss is very great. As a result, almost no beat disturbance is caused by the contiguous track.
A system is capable of recording and reproducing color video signals with a relative narrow frequency band, when a color video signal is separated into a luminance signal and a carrier chrominance signal. The luminance signal is frequency modulated, and the carrier chrominance signal is frequency converted to a band which is lower than the frequency band of the frequency-modulated luminance signal. The resulting frequency-modulated luminance signal and frequency-converted carrier chrominance signal are multiplexed and recorded.
When this system is used with a recording system having azimuth heads, a high frequency band of the frequency-modulated luminance signal can be reproduced without a beat disturbance. However, with regard to a frequency-converted chrominance signal in a low frequency band, there is a small reproducing loss due to the azimuth. For this reason, there is a problem of preventing beat disturbances due to reproduction of contiguous tracks.
Accordingly, the above described problems are resolved in the United States patent application Ser. No. 731,935, entitled Color Video Signal Recording and/or Reproducing System. This color video signal recording system frequency modulates a luminance signal separated from an input color video signal. A first signal processing means converts the frequency of the carrier chrominance signal separated from the input color video signal. The conversion is to a band which is lower than the band of the frequency-modulated luminance signal. The carrier chrominance signal is processed to advance the phase thereof, by 90.degree. for every successive horizontal scanning period, during a specific period. Thereafter, the phase lags by 90.degree. for every successive horizontal scanning period, during a successive specific period. For recording, the frequency-modulated luminance signal and carrier chrominance signal are multiplexed, and recorded on a plurality of tracks which are mutually disposed in parallel on a recording medium. The recording is performed in such a manner that a carrier chrominance signal has a successive phase advancement by 90.degree.. The phase of a carrier chrominance signal lags successively, by 90.degree., in side-by-side disposition on mutual adjacent tracks.
To reproduce a signal recorded by the above described recording system, the frequency-modulated luminance signal is demodulated after it is separated from the reproduced signal. A second signal processing means restores the carrier chrominance signal separated from the reproduced signal, into the original frequency band. The chrominance signal is phase shifted by 90.degree. for every successive horizontal scanning period, to restore the original carrier chrominance signal. The output signal of the second signal processing means is delayed by one horizontal scanning period. The output signal of the second signal processing means is mixed with the output signal of the delay means. The demodulated luminance signal is mixed with the output carrier chrominance signal of the mixing means, to produce a reproduced color video signal.
In this recording and reproducing system, it is necessary to carry out signal processing so as to advance or delay the phase of the carrier chrominance signal by 90.degree. for every horizontal scanning period. Accordingly, for this signal processing, a signal processing circuit is used for producing an output signal wherein the phase shifts sequentially by 90.degree..
However, if the accuracy of the phase shifting of this signal processing circuit is poor, accurate color reproduction cannot be accomplished. For this reason, it is necessary for the error of the 90.degree. phase shift to be within .+-. 1.degree.. Since the carrier chrominance signal is phase shifted by 90.degree. for every one horizontal scanning period (hereinafter represented by 1H), the phase rotates through 360.degree. (that is, returns to its original state) every 4H period. For this reason, if the phase shifting accuracy is poor, color stripes will be generated in the reproduced picture every 4H period.
More specifically, if the phase shifting accuracy is poor, the result will be the same as a phase modulation of the carrier chrominance signal. Moreover, the 4H period in terms of frequency is approximately 3.93 KHz. This frequency is outside the band (normally, 1 - 2 KHz) within which it is possible to remove color phase fluctuation by means of an automatic phase compensator (APC), of the type used in an ordinary recording and reproducing apparatus. Consequently, the phase fluctuation caused by this poor phase shifting accuracy cannot be removed by an APC circuit. For this reason, color stripes will appear in the reproduced picture. Since these color stripes are stationary, they tend to be conspicuous even when they are slight. Accordingly, a strict requisite is required for the phase shifting accuracy of the signal processing circuit.
If this is an analog signal processing circuit using inductances and capacitances, it is subject to drifting due to temperature variation and with the elapse of time. Thus, such a circuit cannot satisfy the accuracy requirement.
As an alternative, the signal processing circuit might use a digital data selector which produces a signal which is phase shifted in a digital manner. By this measure, it is possible to satisfy the above mentioned requirement for phase shifting accuracy. No problem is encountered if a high-speed TTL is used for this digital data selector. However, if a low-speed MOS IC is used for this digital data selector, deviations occur in the duration of a period of time extending from the application of a signal to the address input terminal until the content of the data input terminal appears at the output terminal. As a result, the phase shifting accuracy is poor, and color stripes appear every 4H period in the reproduced picture.